A variety of references have described non-vitreous small particles, e.g., U.S. Pat. Nos. 3,709,706, 3,793,041, 3,795,524, 4,166,147, 4,217,264, and 4,306,913 and U.K. published patent application no. 2,115,799. Some of the small particles described in such references are combined with binders (e.g., polymerizable resins) to form composites, and a few of such small particles may contain sufficient amounts of a suitable ceramic metal oxide to render composites which might be made with such particles opaque to X-ray radiation (i.e., radiopaque).
Radiopacity is a very desirable property for dental composites. Radiopaque composites can be examined using standard dental X-ray equipment, thereby facilitating long term detection of marginal leakage or caries in tooth tissue adjacent to the cured composite. However, a dental composite should also have low visual opacity, that is, it should be substantially transparent or translucent to visible light. Low visual opacity is desired so that the cured dental composite will have a lifelike lustre. If such a dental composite is intended to be cured or polymerized using visible light-induced photoinitiation, then the depth of cure required (sometimes as much as two millimeters or more), the desire for uniform hardness in the cured composite, and the physical limitations imposed by carrying out the curing reaction within the mouth (which require, among other things, that the uncured composite usually be exposed to light from only one angle, and that the curing radiation be provided by a portable instrument) all mandate that the composite have low visual opacity to enable deep, thorough cure.
An additional desirable property for dental composites is durability. Durability sometimes can be improved by increasing the percentage of filler particles in the composite. However, increased filler levels typically lead to increased visual opacity, thus reducing translucency and limiting visible-light cure depth and cure uniformity.
A further desirable property for dental composites is cure stability. For a two-part non-light cure dental composite (i.e., a so-called "chemical cure" composite), cure stability is evaluated by comparing polymerization set times for aged and unaged composite samples when the two parts of the composite are mixed together. For a one-part light-cure dental composite, cure stability is evaluated by comparing top and bottom hardnesses of composite samples that have been aged or are unaged and are cured to a standard depth. Poor cure stability is manifested by a change in polymerization set time or a difference in top and bottom cure hardness between aged and unaged composite samples. The filler particles in a dental composite sometimes interact with the other ingredients of the composite (e.g., with the polymerizable resin or the polymerization initiators) and adversely affect cure stability. These interactions can be unpredictable and are sometimes poorly understood.
In practice, it has proven very difficult to formulate dental composites having a commercially useful combination of high radiopacity, low visual opacity, high filler loading, and cure stability.